1. Field of the Invention
The present disclosure generally relates to charge pumps and more particularly to charge pump circuits that are free of source-sink mismatch.
2. Description of Related Art
Charge pumps are widely used in numerous applications, including frequent use in phase lock loop circuits. As depicted in FIG. 1, a typical charge pump 100 comprises a current source I1, a current sink I2, a first switch S1 controlled by a first logical signal UP, and a second switch S2 controlled by a second logical signal DN. Throughout this disclosure, VDD denotes a first substantially fixed-potential circuit node (usually at a power supply output), and VSS denotes a second substantially fixed-potential circuit node (usually referred to as “ground”). It will also be appreciated that a current sink may also be referred to as a current source (only that it outputs a negative current). When the first logical signal UP is asserted, the current source I1 injects current into circuit node 105 through the first switch S1; when the second logical signal DN is asserted, the source sink I2 drains current from circuit node 105 through the second switch S2. Circuit node 105 is coupled to a load circuit 110 comprising a load capacitor CL, resulting in an output voltage VOUT. An ideal current source (sink) has infinite output resistance and thus will inject (drain) a constant current regardless of a voltage at its output. In practice, however, a real-world current source (sink) has finite output resistance and its output current depends on the voltage at its output. For instance, when the current source I1 (sink I2) is implemented by a PMOS (NMOS) transistor, the magnitude of its output current will decrease (increase) when the output voltage VOUT increases. In many applications, for instance phase lock loop, it is highly desirable that the magnitude of the output current of the current source I1 matches that of the current sink I2. When there is a mismatch (in output current magnitude) between the current source I1 and the current sink I2, the system performance usually degrades.
Therefore, method and apparatus for a charge pump free of the source-sink mismatch is desired.